Plastic optical devices are generally superior to glass-based optical devices having the same constitution in that they have good shapability and workability, they are lightweight, they are inexpensive, they are flexible and they have good impact resistance. For example, as compared with glass-based optical fibers, plastic optical fibers (POFs) are relatively unsuitable to long-distance light transmission since the light transmission loss through them is great, but their advantage intrinsic to the plastic materials constituting them is that the core diameter of the optical fibers may be large, for example, to be tens μm or more. As having such a large core diameter, the plastic optical fibers do not require any superfluous connection accuracy of various peripheral members or units with them in branching or connecting the optical fibers. Accordingly, POFs enable easy connection and terminal working with peripheral members or units, and the cost of the connectors and others for them may be reduced. Still another advantage thereof is that POFs do not require high-accuracy core control. In addition, since POFs are plastic, they has further advantages in that their risk of sticking in human bodies is low, they are flexible and are therefore easy to work and to lay, they are resistant to shock and their cost is low. Accordingly, not only POFs are noticed for household appliances and vehicles, but also they are being investigated for their application to internal wiring in high-speed data processors and to ultra-short-distance and large-capacity cables for DVI (digital video interface) links.
In general, POF comprises a core and a clad, in which the clad is an outer shell and the core fills the space inside the clad, and the refractive index of the core is higher than that of the clad. Recently, a refractive index profile POF, in which the core has a refractive index profile that varies from its center to its outside, has become specifically noticed as an optical fiber having a high transmission capacity. One method known for producing the refractive index profile POF comprises preparing an optical fiber preform and then melt-stretching the preform.
In preparing the preform, a polymerizable compound to form a core is put into a pipe for a clad, the clad pipe is put into a tubular chamber, and the core is formed therein while the tubular chamber is rotated. During the core formation, the polymerizable compound in the core-forming composition is polymerized so as to form the core tubularly successively from the side of the clad pipe to the center thereof, while the content of a compound for refractivity control in the composition is varied toward the center of the pipe. The core thus formed according to the polymerization method has a concentration profile of the refractivity controlling-agent contained therein, and accordingly the core has a refractive index profile. The preform thus formed is stretched at a predetermined temperature to give a refractive index profile, plastic optical fiber (e.g., see JP-A 5-173025, 8-262240, 2001-215345).